Arc lamp construction

ABSTRACT

An arc lamp construction in which the lamp envelope portion surrounding the main body portions of the arc electrodes is spaced therefrom to define a cylindrical gap and in which a packing member is disposed in the cylindrical gap to support the electrodes. The packing member is elastic, or radially deformable, to prevent cracking of the envelope as the gap width changes with temperature, and is thermally conductive to allow the transfer of electrode heat across the gap to the envelope.

ilnited States Patent Inventor Raymond E. Paquette Saratoga, Calif.Appl. No. 871,201 Filed Oct. 13, 1969 Patented Dec. 28, 1971 AssigneesRepublic National Bank of Dallas;

lrving Trust Company; Union Bank Original application Oct. 24, 1967,Ser. No. 677,565, now Patent No. 3,518,480, dated June 30, 1970. Dividedand this application Oct. 13, 1969, Ser. No. 871,201

ARC LAMP CONSTRUCTION 4 Claims, 10 Drawing Figs.

U.S. Cl 29/25.16, 29/25.l3, 316/19 Int. Cl l-l01j 9/36, H0 1 j 9/18Field of Search 29125.1,

25.11,25.l3,25.l5,25.l6;3l6/19,24,30

Primary Examiner-Charlie T. Moon Assistant ExaminerRichard BernardLazarus Anorneys- Harvey G. Lowhurst and Claude A. S. l-lamrickABSTRACT: An arc lamp construction in which the lamp envelope portionsurrounding the main body portions of the arc electrodes is spacedtherefrom to define a cylindrical gap and in which a packing member isdisposed in the cylindrical gap to support the electrodes. The packingmember is elastic, or radially deformable, to prevent cracking of theenvelope as the gap width changes with temperature, and is thermallyconductive to allow the transfer of electrode heat across the gap to theenvelope.

PmENTifinitewfl 3529.915

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RAYMOND E. PAQU ETTE 1 Lkikr ARC LAMP CONSTRUCTION This application is adivision of my copending application Ser. No. 677,565, filed Oct. 24,1967, now U.S. Pat. No. 3,518,480.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to are lamps, and, more particularly, to an arc lampconstruction that is capable of withstanding severe shock and vibrationby the provision of an electrode support in the nature of a soft packingthat also aids in maintaining precision alignment of the electrodes andimproves the heat transfer from the electrodes to the envelope, andthereby lengthens the useful operating life.

For certain arc lamp applications the arc lamp is exposed to anenvironment which makes extremely high demands on the mechanical andelectrical integrity of the lamp. For example, in missile and rocketapplications, an arc lamp has to withstand severe shock and vibration,and consequently requires very firm anchoring of its electrodes toprevent electrodes from shaking loose and to maintain a constant arcgap. Further, since weight and space are always problems in missile androckets, a light and small arc lamp is always desired. Since therequired useful lifetimes are usually short, such are lamps can be runabove their normal rating for short times, thereby allowing a muchlighter and smaller arc lamp to be used.

The major problem encountered in arc lamp constructions which have goodsurvival capabilities to vibration and shock, and which may beoverdriven to provide a minimum required amount of illumination aboveits normal rating, is that the refractory electrode material has acoefficient of expansion which is about eight times greater than theradiation transparent envelope material. Accordingly, to securelysupport the electrodes within the envelope requires the envelope to betightly or hard-shrunk upon the electrode body. However, if this is donewhen the electrode is cold, the subsequent heating of the electrodesduring arc operation will fracture the envelope because of the unequalrate of expansion. If the envelope is hard-shrunk upon the electrode toprovide a tight fit at normal operating temperatures, then theelectrodes will be loose prior to reaching their normal operatingtemperature, and will be subject to destruction when exposed to shockand vibration. Additionally, it is almost impossible to controlmanufacturing tolerances so closely that a good tight fit between theenvelope and the electrode at a usually ever-increasing operatingtemperature can be accomplished. This is particularly true since heatdissipation from the electrode to the envelope is best when the contactis good.

2. Description ofthe Prior Art One type of arc lamp which isparticularly suited for missiletype applications is the self-startingshort are lamp which utilizes a bridge wire across the arc gap which isexploded to start the lamp. This lamp is described in U.S. Pat. Nos.3,256,459 and 3,274,427 and in copending Pat. application Ser. No.554,933, filed May I9, 1966, titled Self Starting Arc Lamp" and assignedto a common assignee.

Even though a bridge wire short arc lamp performs satisfactorily, itssurvival capability, arc stability, and useful like duration, whenexposed to severe shock, vibration, and power overloads, has been foundwanting in some respects. For example, when the lamp is constructed towithstand shock and vibration when cold (no arc), there is always dangerof envelope cracking when the lamp is operating because of electrodeexpansion. When the lamp is constructed to withstand shock and vibrationwhen hot, shock and vibration damage prior to reaching operatingtemperatures are common, and, additionally, the heat dissipation ispoor, thus shortening useful life.

Even though this invention will be explained with particular referenceto self-starting short arc lamps, it is to be understood that it isequally applicable to ordinary arc lamps, whether short or long arc, andwhether started with a firing pulse or a bridge wire. The presentinvention can be employed wherever there are problems of shock,vibration, electrode alignment, or heat transfer in lamps in whichelectrodes are usually cantilevered into the bulb from the electrodefoot.

SUMMARY OF THE INVENTION It is therefore a primary object of theinvention to provide an arc lamp having improved electrode supports andimproved electrode heat dissipation for greater shock and vibrationsurvival capacity and longer useful life of the lamp.

It is another object of the invention to provide an improved electrodesupport for facilitating and maintaining precision alignment of theelectrodes in an arc lamp.

It is a further object of the invention to provide structure between aportion of an electrode and the inner periphery of the lamp envelope forimproving the heat transfer characteristics of an arc lamp therebyproviding longer life for the lamp.

It is still another object of the invention to provide a flexible orresilient or elastic or radially compressible and expandable or softelectrode support packing for an arc lamp resulting in an improved shockand vibration survival capability, a better alignment of the electrodes,and an improved heat transfer from the electrode to the envelope.

It is another object of the invention to provide a resilient electrodesupport for facilitating manufacture of the arc lamp to prevent breakagethereof.

It is a further object of the invention to provide a method ofmanufacture of an arc lamp wherein flexible resilient electrode supportpacking is utilized to compensate for the different coefficients ofexpansion of the electrodes and the envelope.

Briefly, one embodiment of an arc lamp of this invention accomplishesthe stated objects by utilizing a helical coil of molybdenum wire woundaround a portion of the exterior surface of each electrode forapproximately eight turns. The wire coil is in the form of a helix or aspiral and has a fomied interior diameter which is slightly smaller thanthe diameter of the electrode so that the coil is, in effect, sprungover the electrode for securement thereto. In the alternative, and forgreater securement, the wire coil may also be affixed to the electrodeat a single point by a spot weld or the like.

The interior surface of the envelope is ridged or grooved around theindividual coils of wire to prevent dislodgement in a direction alongthe axis of the lamp. When the lamp is cold, there is a very small gapbetween the coil and either the external surface of the electrode or theinternal surface of the envelope to allow for expansion. When the coilof wire becomes heated during operation of the arc lamp, the heatingcauses the coil of wire to expand as by winding along the longitudinalextent of the electrode. With the coil of wire being flexible orresilient, and having the ability to wind and unwind in the peripheralspace between the exterior surface of the electrode and the interiorsurface of the envelope, the electrode is thus flexibly and resilientlysupported within the envelope both when hot and cold, and the likelihoodof breakage of the envelope by heating or from vibration and shock isgreatly reduced. Also, the juxtaposition of the coil relative to theelectrode facilitates alignment of the electrodes both during assemblyof the arm lamp and while the lamp is in use. Further, the coil of wiresurrounding the electrodes, being metallic and in contact with both theelectrode and the envelope, improves the heat transfer from theelectrode during arcing of the lamp and thus prolongs the life of thearc lamp.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a view in elevation of aself-starting short arc lamp constructed in accordance with thisinvention;

FIG. 2 is an enlarged view, partly in elevation and partly in crosssection, of a portion of the arc lamp of FIG. I illustrating theelectrode with a coil wire packing member;

FIG. 3A is a view, partly in elevation and partly in cross section, of amodification of the invention;

FIG. 3B is a view taken along lines 38-38 of FIG. 3A;

F16. 4A is a view, partly in elevation and partly in cross section, of afurther modification of the invention;

FIG. 4B is a view taken along lines 4B-4B of FIG. 4A;

FlG. 5A is a view, partly in elevation and partly in cross section, ofanother modification of the invention;

FIG. 5B is a view taken along lines 58-58 of FIG. 5A;

FIG. 6A is a view, partly in elevation and partly in cross section, ofstill another modification of the invention; and

FIG. 6B is a view taken along lines 6B6B of FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,in which like reference numerals designate like parts, and particularlyto FIGS. 1 and 2, a short are bridge wire lamp 10 is shown constructedin accordance with the invention. Lamp 10 includes a lamp envelope 12,usually made of quartz for well-known reasons which has typically athermal coefficient of linear expansion of about 4.9 10 per C. Mountedwithin lamp envelope 12 are a pair of electrodes 13 and 14 whichrespectively form the anode and cathode of lamp 10, and which are madeof a refractory metal such as swaged tungsten which typically has athermal coefficient of linear expansion of about 42x10 per C. Electrodesl3 and 14, respectively, include a main body portion 15 and 16, footportions and 21, and are end portions 35 and 36. Electrodes 13 and 14may be identical, as shown in the drawings, so that either may beselected as the cathode thereby making the lamp nonpolarized. Of course,if desired, one electrode may be constructed of a larger diameter thanthe other to form the hotter anode as is usually done in arc lamps.However, the fact that the lamp may be made nonpolarized is an advantagein that savings are made in manufacturing cost, and the user can connectthe voltage supply to the lamp without regard to polarity. 1f lamp 10 isof the self-starting type, a bridge wire 50 is connected across theelectrode gap.

Surrounding body portions 15 and 16 of the electrodes are a pair ofwires 17 and 18 which, in the preferred embodiment, are made ofmolybdenum having a thermal coefficient of linear expansion of about5OX1O" per C. Molybdenum wires 17 and 18 are formed into helical coilsof approximately six to eight turns, and the coils, prior to beingplaced on the electrodes, have an internal diameter which is slightlysmaller than the diameter of the electrode body portion. The coils are,in effect, sprung over the electrode body portion to hold them in place.For additional assurance, the wire coils may also be secured to theelectrodes at a single location, as at 19 in FIG. 2, to allow theremainder of the coil to wind and unwind over the exterior surface ofthe electrode body portions, as will be described hereinafter.

Electrodes 13 and 14 are primarily supported, in cantilever fashion,within envelope 12 by shrinking the portion of envelope 12 coextensivewith electrode feet 20 and 21 about the electrode feet. Wire coils 17and 18, as will become clearer from the ensuing description, supplementthe cantilever support and further secure the electrodes againstlongitudinal and lateral displacement when the envelope portioncoextensive with the wire coils is shrunk about the wire coils.

This shrinking is accomplished as an additional step in the manufactureof the lamp after the envelope has been shrunk around electrode feet 20and 21 and sealed over the ribbons. More particularly, the arc lamp,which is under negative pressure for well-known manufacturing reasons,is heated uniformly to a temperature which corresponds to its operatingtemperature which is between 1,400 and 1,500 C. This causes theelectrodes, the wire coils, and the envelope to expand in accordancewith their respective thermal coefficients of linear expansion andassure their sizes at operating conditions.

Thereafter, the portion of envelope l2 coextensive with wire coils l7and 18 is heated to about 1,650 C. which is close to the softening pointof the quartz. Because of the negative pressure inside envelope 12, theheated portion, now softened, moves inwardly under the negative pressureuntil it comes into contact with wire coils l7 and 18. This correspondsto a hard shrink of the envelope upon the wire coils. The degree ofpenetration of envelope 20 into the spaces between the coils iscarefully controlled by the flame so that good-sized corrugations orgrooves 22 are formed in the interior surface of the envelope. After adesirable degree of penetration is obtained, the arc lamp envelope isallowed to cool which causes electrodes 13 and 14, coils 17 and 18, andenvelope 12 to contract. Since the thermal coefficient of linearexpansion of the quartz is approximately 10 times less than that of thetungsten electrode, or the molybdenum wire, it is readily seen thatcoils 17 and 18 will shrink away from the quartz envelope and therebychange the hard shrink support to a soft support. For a wire coil, whichis typically constructed of 0.0l5-inch diameter wire, and whichsurrounds an electrode having a diameter of about one-eighth of an inch,it has been found that the shrink-away is about 0.001 inch.

It is to be noted at this point that whenever the arc lamp is at itsoperating temperature, a hard shrink condition exists between theenvelope, the wire coils and the electrodesv This condition is notdangerous to the integrity of the envelope since the envelope was shrunkon the coil-electrode combination at this temperature and the forces,due to expansion of the electrodes, are not in excess of those whichcaused the envelope to shrink about the electrode in the first placeduring manufacture. Accordingly, during operating condition, there isfirm lateral and longitudinal support of the electrodes and good thermalcontact from the electrodes to the envelope which is due to thehard-shrunk condition.

When the arc lamp is cold, wire coils 17 and 18 become elastic springswhich resiliently support the electrodes within the envelope. As wirecoils l7 and 18 contract during cooling, they become shorter and wind upupon the electrode body using grooves 22 as guides which they threadablyengage. This is best seen in FIG. 2 where wire end 19 may be secured tothe electrode, and wire end 19a is movable through a distance D duringwinding and unwinding. When the lamp 10 is cold, it can be said that theelectrodes are flexibly or resiliently supported by the coils of wirewithin the envelope. Also, it can be seen that the coils of wires 17 and18 help maintain the electrodes 13 and 14 in alignment both duringassembly and during use and, most importantly, if the coils of wires arefirmly secured to their respective electrodes, then grooves 22 securethe electrodes against axial displacement.

Foot 20 of electrode 13 is connected to a pair of conductive ribbons 2Sand 26 by spot welds 10 tantalum or molybdenum wire (typically 0.006inch) tabs 27 and 28, one on each side of electrode foot 20 as shown inFIG. 1. Ribbon 25 is typically made of molybdenum having a width ofone-eighth of an inch and a thickness of 0.7 mils. The other end ofribbon 25 is similarly connected to a terminai lug 30 to which aflexible lead 31 is affixed. The advantage of having a pair of ribbons25 and 26 at the electrode foot is that a layer of metal ribbon is thussuperimposed between the electrode foot and the quartz envelope. Suchsuperimposed layer of metal ribbon helps prevent breakage as the quartzenvelope is shrunk in relation to the interior lamp assembly justdescribed.

A similar metal ribbon construction is connected to the other electrodeand to a terminal lug 32 to which flexible lead 33 is attached. Thislamp construction is generally known as a ribbon seal lamp since theseal between the lamp interior and the outside is formed around theribbons. it is to be understood that the present invention is equallyapplicable to rod seal" lamps in which the ribbons are replaced by rodsand where a seal is formed around the rods.

Thus, the components of a short are bridge wire lamp have been describedwherein a flexible resilient electrode support feature has beendescribed in conjunction with coils of wire 17 and 18 surrounding bodyportions 15 and 16 of electrodes 13 and 14.

Before describing a number of modifications of the electrode supportshown in FIGS. 1 and 2, it may be helpful to delineate the generalsupport requirements which are met by the wire coils. As a consequenceof the cantilever support of the electrodes in the lamp envelope, thebasic survival capability of the arc lamp to severe shock and vibrationis low. Shrinking the envelope upon the body portion of the electrodewhen the same is below operating temperature is not possible because theelectrode has a much greater thermal coefficient of linear expansionthan the envelope and would, therefore, crack the envelope whenexpanding during arc operation. Shrinking the envelope directly upon thebody portion of the electrode when the same is at operating temperatureis likewise not the answer because, when the lamp is cold, the electrodeis not securely supported against lateral and longitudinal displacement.Accordingly, what is required is a soft support when the lamp is coldand a hard support when the lamp is hot, the hard support correspondingto shrinking the envelope upon the electrode.

In accordance with this invention, a cylindrical space 51 is providedbetween the exterior surface of the main body of the electrode and thecorresponding interior surface of the envelope. Disposed in this spaceis a packing with certain properties. The packing substantially fillsthe cylindrical space to thereby support the electrode within theenvelope at any electrode temperature. The packing provides a thermalpath across the space to allow for the dissipation of heat. Finally, thepacking not only provides lateral support by filling the space, but mayalso provide longitudinal support by affixing the packing to theelectrode and shaping the envelope as is obtained by the coils of wiredescribed in connection with FIGS. 1 and 2 which are secured to theelectrodes and threaded in grooves 22.

The above characteristics of the packing are provided by a metallicpacking which is radially compressible and expandable to accommodateitself to the variation of the radial gap dimensions due to theenormously different thermal coefficient of linear expansion. The coilsprings meet these requirements since they are elastic and in contactwith the electrode and the envelope when cold or hot. When hot, theclearance disappears and good support and thermal conductivity by ahard-shrunk fit is assured. With these basic concepts in mind, it willnot be understood that a large number of modifications of the packingare possible without departing from the spirit of the present invention.

FIGS. 3A and 3B disclose a modification of the invention wherein thepacking member about which the envelope 12 is shrunk takes the form of ametallic refractory ribbon spring 52 which is wound several times aroundbody portions of electrode 13. The interior end 53 of the metallicribbon spring 52 is secured to the electrode as by spot welding or thelike, while the other end 54 is allowed to remain free. Thus, the coilof metallic ribbon 52 is free to wind or unwind relative to the bodyportion of the electrode and within the deformed position of theenvelope shrunk thereabout as the ribbon expands and contracts as aresult of ambient conditions. The metallic ribbon 52 thereby resilientlysupports the electrode 13 to withstand vibration and shock by thecushioning action of the wrapped ribbon and the freedom of expansion andcontraction of the ribbon 52. Furthermore, the ribbon engages the edge49 of the annular groove formed in the envelope 12 as it was shrunkthereabout to provide the desired longitudinal integrity. The metallicribbon 52 wound around the body portion 15 also affords the advantagesof improved heat transfer from the electrode to the envelope, and aidsin maintaining the electrodes in alignment.

FIGS. 4A and 4B disclose a further modification of the invention whereinthe packing member takes the form ofa corrugated cylinder 60 havingridges 61 in contact with the interior surface of envelope 12 and trough62 in contact with body portion 15 of electrode 13. To secure packingmember 60 to electrode 13, portion 15 may be threaded or a portion suchas 62 may be spotwelded to electrode 13. Likewise, for longitudinalintegrity, envelope 12 may be shrunk over corrugated cylinder 60 in amanner similar to that described in connection with shrinking theenvelopes over the coils of wire, Elasticity,

or radial compressibility, provides the necessary adjustment for thevariation in the gap distance,- and thereby supports electrode 13 eitherin the hot or cold condition. Further, good thermal transfercharacteristics are associated with a corrugated cylinder 60. FIGS. 5 Aand 5B show a still further modification of the invention wherein thepacking member takes the form of a lengthwise corrugated cylinder 70having crest at 71 for engaging the interior surface of envelop l2 andvalley 72 for engaging body portion 15 of electrode 13. FIGS. 6A and 6Bdisclose yet another modification of the invention wherein the packingmember takes the form of a cylinder which includes a number of outwardlyprojecting spaces 81 for engaging the interior surface of envelope 12.The interior surface 82 of cylinder 80 may be shrunk over body portion15 of electrode 13.

In the embodiments shown in FIGS. 3A, 4A, 5A, and 6A, longitudinalsupport may be provided by shrinking envelope 12 over the points ofcontact with the packing member. Lateral stability is usually providedby the two point contact provided by different surfaces of the packingmember with the interior envelope surface and exterior electrodesurface.

Thus, an arc lamp having flexible resilient electrode supports has beendescribed wherein damage from vibration and shock is reduced, heattransfer is improved, and alignment of the electrodes is facilitated andmaintained, thereby improving its overall operation of the arc lamp andits longevity.

While the above-detailed description has shown, described, and pointedout the fundamental novel features of the invention as applied tovarious embodiments, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated may be made by those skilled in the art, without departingfrom the spirit of the invention. It is the intention, therefore, to belimited only as indicated by the scope of the following claims.

What is claimed is: 1. In the method of constructing an arc lamp havinga pair of cantilever supported, spaced-apart, refractory electrodesdisposed within an envelope, the improvement comprising the steps of:

winding a coil of refractory wire around a portion of at least one ofsaid electrodes prior to placing the same in the envelope, thewire-wound portion being disposed on the side of the cantileveredportion adjacent the arc end of the electrode; heating the sealedenvelope, with the electrodes in place and the envelope under negativepressure, to the normal arc lamp operating temperature; and

shrinking the portion of the envelope opposite said coil of wire withoutwetting on said coil or wire for lateral support of the electrode.

2. The method in accordance with claim 1 which includes the additionalstep of forming a spiral groove in the interior surface of the portionof the envelope being shrunk upon the coil of wire to partiallyaccommodate the coil of wire for longitudinal support.

3. The method of constructing an arc lamp having a pair of cantilevered,spaced-apart electrodes disposed within an envelope, said methodcomprising the steps of:

winding a coil of refractory wire around the normally unsupportedportions of each of said electrodes prior to placing the electrodeswithin the envelope;

sealing the envelope under a reduced internal pressure andhard-shrinking the envelope about the electrodes feed adjacent thenormally unsupported portion of the electrode on the side opposite thearc end to provide the cantilevered support;

heating the envelope, electrodes and coils of wire to the normal arclamp operating temperature with a negative pressure inside the envelopeto shrink the envelope without wetting about said coils of wire toprovide electrode body support at the normal arc lamp operatingtemperature.

4. A method in accordanE' ivlth claim 3 in which the step of shrinkingthe envelope about the electrode feed follows the step of shrinking theenvelope about the coils of wire.

t I F Q!

1. In the method of constructing an arc lamp having a pair of cantileversupported, spaced-apart, refractory electrodes disposed within anenvelope, the improvement comprising the steps of: winding a coil ofrefractory wire around a portion of at least one of said electrodesprior to placing the same in the envelope, the wire-wound portion beingdisposed on the side of the cantilevered portion adjacent the arc end ofthe electrode; heating the sealed envelope, with the electrodes in placeand the envelope under negative pressure, to the normal arc lampoperating temperature; and shrinking the portion of the envelopeopposite said coil of wire without wetting on to said coil or wire forlateral support of the electrode.
 2. The method in accordance with claim1 which includes the additional step of forming a spiral groove in theinterior surface of the portion of the envelope being shrunk upon thecoil of wire to partially accommodate the coil of wire for longitudinalsupport.
 3. The method of constructing an arc lamp having a pair ofcantilevered, spaced-apart electrodes disposed within an envelope, saidmethod comprising the steps of: winding a coil of refractory wire aroundthe normally unsupported portions of each of said electrodes prior toplacing the electrodes within the envelope; sealing the envelope under areduced internal pressure and hard-shrinking the envelope about theelectrodes feed adjacent the normally unsupported portion of theelectrode on the side opposite the arc end to provide the cantileveredsupport; heating the envelope, electrodes, and coils of wire to thenormal arc lamp operating temperature with a negative pressure insidethe envelope to shrink the envelope without wetting about said coils ofwire to provide electrode body support at the normal arc lamp operatingtemperature.
 4. A method in accordance with claim 3 in which the step ofshrinking the envelope about the electrode feed follows the step ofshrinking the envelope about the coils of wire.